Rotary mechanical actuators have been utilized to fold wings of military aircraft in a unique folding geometry which permits a smaller actuator package to provide an effective higher torque moment combined with maximum extension. In military aircraft such as, for example, A-6 or A-12, a wing fold geometry having a fold angle of about 167.degree. is required, with the rotary actuator utilizing counter rotating output arms combined with linkages that can extend to more than twice an original envelope, that is, the actuator arms rotate 366.degree. for a 167.degree. rotation of a wing resulting in approximately a two-to-one mechanical torque advantage, thereby resulting in a lower weight since weight of the actuator is in proportion to a torque.
Rotary actuator systems generally include a valve, hydraulic motor, hydraulic brake, simple and differential planetary gear assembly as well as a stop means.
The differential planetary gear assembly for the rotary actuator system generally incorporates two different gear ratios, with the first ratio resulting in a negative value between large arms of the actuator and the center portion of the actuator while a second ratio results in a positive value between the small arms and center portion of the actuator. Consequently, in the proposed rotary actuator systems, with respect to the center portion of the actuator, the smaller side arms rotate in one direction while the larger arm rotates in an opposite direction, with the gears in the differential planetary gear arrangement being cageless and balanced.
Moreover, geared rotary actuator generally only have one mesh on either side of a center gear mesh. During assembly, the planet gears are positioned around the support rings, and the ring gears are slid into place over the planet/support ring assembly. To facilitate assembly, a pitch diameter on the center mesh is larger than a pitch diameter of the end mesh.
If an additional gear mesh is added to one side of the center mesh, assembly problems arise. More particularly, if the inboard mesh pitch diameter must, for some reason, be smaller than the outboard mesh and center mesh, the inboard mesh ring gear could be prevented from sliding over the outboard planet gears due to the larger outboard pitch diameter.
One disadvantage in planetary gear arrangements for rotary actuator systems of the aforementioned type resides in the fact that the planetary gears are splined in the actuator. The splines lengthen the actuator to provide a sufficient torque carrying capability; however, lengthening of the rotary actuator adds to the weight. Additionally, the provision of splined connections is expensive by virtue of the required machining operations for the planetary gears and the spline shaft.
U.S. Pat. No. 4,721,016 proposes a multi-staged geared rotary actuator for positioning aircraft flight control surfaces such as, for example, leading edge flaps, wherein a support of axially-aligned tubular planet gear shafts in successive stages is achieved by means extending through the axially-aligned tubular planet gear shafts which supports the ends of the planet gear shafts and reacts against bending forces applied to the ends of the tubular gear shafts to reduce deflection and maximize gear mesh of gears carried on the planet gear shafts for maximum torque transmission.
While the above-proposed multiple-stage geared rotary actuator is effective for advantageously controlling a positioning of aircraft flight control surfaces, the proposed actuator is not faced with any assembly limitations in spite of the use of a multi-partite planetary gear construction.
A compound gear arrangement is proposed in U.S. Pat. No. 4,751,855, also intended for use as a geared hinge for aircraft, each comprising a plurality of substantially axially aligned gear trains each having two relatively rotatable output ring gears surrounding a sun gear input element and two planet gear elements, with the planet gear elements being coupled for rotation in unison by the sun gear element and meshing with respective ones of the ring gears. The sun gear elements of adjacent gear trains are drivingly coupled by a means which permits an axial misalignment between the sun gear elements.
While the above-proposed compound gear arrangement solves a flexibility problem, by virtue of the proposed constructional features, no particular assembly problem exists in the proposed compound gear arrangement.